In a color television CRT display, the display screen on a face of the tube is provided with a matrix of grouped phosphor areas wherein three phosphors, stripes or the like, define each unit of the display. Each of the stripes in a group emits light in one of the three primary colors when excited by an electron beam. An electron gun structure is disposed at one end of tube, and three electron beams are directed through a shadow mask onto the screen phosphors. The shadow mask has a matrix of openings aligned with the phosphor areas and with the electron gun structure such that when the three electron beams are directed at a particular phosphor group defining a unit of the display, the electrons from each beam impinge only on that section of the grouped phosphors containing the phosphors for the color associated with that particular beam. The shadow mask blocks the nearby phosphor areas (which correspond to other colors) from the electrons emitted by that particular beam. These nearby areas instead are aligned relative to the respective opening in the shadow mask to receive only electrons from their respective beams.
Electrons for exciting the screen phosphors are electrostatically accelerated from the electron gun structure to the screen. The beams from all three guns are scanned to form a raster when passing over the grouped phosphors. The scanning is accomplished by magnetic fields varying at the horizontal and vertical scanning rates, produced by deflection coils disposed on the tube adjacent the electron gun structure. The electron gun structure is at one end of a funnel shaped tube having a narrow neck, and the deflection coils are disposed along the neck of the tube. The deflection coils produce magnetic fields aligned vertically for horizontal (X axis) deflection and horizontally for vertical (Y axis) deflection, relative to a center line of the electron beams, which defines a Z axis. The moving electrons comprise a current, and when subjected to the magnetic fields of the deflection coils, the electrons are accelerated in the X and/or Y directions and follow a curving path.
Ideally, the deflection coils occupy a limited extension and have relatively confined fields along the Z axis. Therefore, the electrons are vertically and/or horizontally accelerated for deflection purposes in a limited area. After emerging from the deflection field region, the electrons pass in a substantially straight line from the deflection coils to the point at which the electrons impinge on the screen phosphors through the shadow mask.
Insofar as after such deflection the electrons passing between the deflection coils and the screen are subjected to ambient magnetic fields, they are likewise accelerated by such fields and follow a curving path leading toward the screen. The extent and direction of curving depends on the flux density, source and orientation of the further magnetic fields. The deflection of the electron beams by the further magnetic fields, and the curvature of the electron beam path thereby produced, can be such as to misalign the electron beams relative to the shadow mask. As a result, electrons from a given electron beam may impinge partly on the color phosphors adjacent their intended color phosphors rather than only on the intended color phosphors, producing a deterioration of the purity of displayed colors.
Some magnetic fields affecting electron beam deflection are due to the unwanted magnetization of magnetically permeable elements of the display apparatus. The shadow mask may be steel. A steel rim is normally provided around the periphery of the faceplate including the screen, for protection against implosion of the picture tube in the event of breakage. An internal magnetic shield along the funnel shaped surface of the tube between the deflection coils and the screen face plate may also be provided, and is a magnetically permeable element. If these elements become magnetized, color purity is affected. Furthermore, magnetization of the internal magnetic shield can affect its permeability and decrease its effectiveness to isolate the electron beam path from incident fields such as the earth's magnetic field.
To overcome magnetization of permeable elements of the display, the prior art has conceived of field cancellation apparatus operable to apply to the tube a countervailing DC magnetic field. The apparatus may use a coil placed on the surface of the tube, having a loop or pair of loops disposed to encompass a portion of the tube. The coils, for example, can extend to a point near the protective steel rim, and extend rearwardly over the surface of the tube to encompass the magnetic shield within the tube. Specifically, a larger upper coil slopes forward over the top of the tube funnel, and a smaller lower coil slopes forward along the bottom. A DC current is applied to the coils, to counter effects of the geomagnetic field. These coils are substantially as wide as the tube, and thereby encompass the protective rim and the like as well as the contents of the tube generally, including the internal shield. Other forms of coils are also known, for example, wherein the coil simply encircles the tube immediately behind the screen.
The earth's magnetic field is an ambient field having horizontal and vertical components tending to deflect the electron beams so as to adversely affect color purity and the shape of the raster. Depending on the orientation of the CRT Z axis relative to the earth's magnetic poles, the geomagnetic field components may deflect the beams horizontally or vertically (due to vertical and horizontal field components, respectively) and will produce a twist of the raster (i.e., rotation of the beams about the Z axis).
The earth's field is not great in magnitude relative to the magnitude of the deflection coil fields; however, the earth's field accelerates and deflects the electron beams over a relatively long span as compared to the span of the deflection coils. Accordingly, there is a need to reduce the effect of the geomagnetic field.